ALP is present in external secretions such as bronchial mucous, saliva, seminal fluid, cervical mucous and nasal discharge, and it is a serine protease inhibitor with a molecular mass of 12 kDa consisting of 107 amino acid residues, which is considered to be the same substance as secretory leukoprotease inhibitor (SLPI), bronchial mucous inhibitor, mucous protease inhibitor and human seminal inhibitor. The amino acid sequence of ALP has been determined (WIPO Publication No. WO86/03497), and the protein's gene has been isolated and sequenced (WIPO Publication No. WO86/03519).
From homology comparisons, ALP is known to comprise two inhibitor domains. One is an N-terminal domain which is postulated to inhibit various trypsin-like enzymes, and the other is a C-terminal domain which from X-ray crystal structure analysis research is believed to bind to chymotrypsin and has an elastase-inhibiting activity. M. G. Gruetter, The EMBO. Journal, Vol.7, No.2, pp.345-351 (1988)!.
Since ALP inhibits chymotrypsin-like enzymes such as leukocyte elastase and cathepsin G and also inhibits trypsin-like enzymes such as trypsin, plasmin, kallikrein and thrombin, etc., it is known to be connected with emphysema, arthritis, glomerular nephritis, periodontitis, muscular atrophy, tumor invasion (WIPO Publication No. WO86/03497), chronic bronchitis, and chronic cervical inflammation (Japanese Unexamined Patent Publication No. 62-259591). However, the role of ALP in viral diseases is still unknown.
Viral infections occur through the steps of 1) attachment of a virus to membrane receptors on the target cells, 2) membrane fusion between the virus envelope and the target cell membrane, and 3) transfusion of the viral genome into the target cells. In the step 2) of membrane fusion, viral envelope glycoprotein precursors have to be converted to the mature form of virus envelope glycoproteins with membrane fusion activity. After this conversion by proteolysis, the virus acquires the activity of membrane fusion between the virus envelopes and cell membranes in the respiratory-tract. Hemagglutinin (HA) of influenza virus and F.sub.0 of Sendai virus (Paramyxovirus parainfluenza virus Type I) are the envelope glycoprotein precursors, and their proteolytic cleavage is essential for expression of viral infectivity and for the viral replication.
The present inventors have succeeded in isolating a novel arginine-specific serine protease from rat lungs, which has been named "tryptase Clara" (The Journal of Biological Chemistry, Vol.267, pp.13573-13579, 1992).
Tryptase Clara cleaves the F.sub.0 of Sendai virus into two subunits F.sub.1 and F.sub.2, and activates infectivity of the virus in vitro in a dose-dependent manner. In addition, antibodies against tryptase Clara are known to inhibit the proteolytic activation of Sendai virus in rat lungs, resulting in the suppression of the viral replication and pathological changes in rat lungs (Journal of Virology, Vol.66, pp.7211-7216, 1992).
Furthermore, tryptase Clara also cleaves the HA of influenza A/Aichi/2/68(H3N2) virus into HA.sub.1 and HA.sub.2 (The Journal of Biological Chemistry, Vol.267, pp.13573-13579, 1992).
Taken together, tryptase Clara is believed to be a principal host factor which determines the pathogenicity of such contagious viruses in the respiratory-tract.
The present inventors have previously discovered that pulmonary surfactant inhibits the cleavage by tryptase Clara of virus envelope glycoprotein precursors, and thus blocks the viral infection of bronchial mucous epithelial cells and viral replication (WIPO Publication No. WO94/00181, FEBS Letters, 322, 115-119 (1993)).